1. Field of the Invention
The present invention relates to a medium density structural flakeboard lamina and more specifically to a board using such a lamina made from a mixture of hardwood species wherein the flakes are sized and oriented randomly or aligned along the length or width of the lamina and the resin is a phenol formaldehyde resin having a major quantity of a low molecular weight fraction. A medium density structural flakeboard lamina according to the invention may be used as the core of a structural board having veneer, hardboard or plywood face panels, or three or more lamina may be laid up to form a board wherein the flakes in the face lamina are oriented along the length of the panel and the flakes in the core lamina are oriented across the length of the panel or randomly oriented.
2. Prior Art
For many years, manufactured board products have been used in the form of panels for structural purposes, including roof and wall sheathing and flooring. Originally, such panels were made from plywood, but, in more recent years, consideration has been given to composite boards formed from wood dust, particles, strands, or flakes. This consideration results, in part, from the shortage of high quality peeler logs suitable for the veneer used to make plywood. Further, there has been a desire to use the waste materials from sawmills and plywood plants as the furnish for saleable composite boards. More recently, attempts have been made to use the hardwood species which are found on the softwood timber sites.
The Clark and Mottet U.S. Pat. No. 2,689,092 (1954) disclosed a method and apparatus for forming cross-cut flakes from softwood lumber, such as split logs, small roundwood, and sawmill trim. Cross-cut flakes were formed by positioning the edge of the cutting knife parallel to the wood fiber and moving it during the cutting operation normal to the fiber so as to avoid the breakage and curling of the fiber characteristic of planer shavings where the cutting action is in the direction of the fiber. Such cross-cut flakes were used by Clark and Mottet for boards or panels in which the flakes were oriented in a random manner in the plane of the board face. Boards formed in this way were characterized by essentially uniform physical properties in both the length and width dimensions. See Clark U.S. Pat. Nos. 2,773,789 (1956) and 2,773,790 (1956). The product of the Clark and Mottet process was known as a "flakeboard" and utilized waste softwood, typically Douglas fir, and 2 to 4 percent of a phenol formaldehyde, urea formaldehyde, or melamine resin consolidated to a specific gravity of about 1 (i.e., a density of about 64 pounds per cubic foot) on an oven dry weight and volume basis. Although the product was generally comparable in strength to plywood, the density was high and well outside the range of medium density products (e.g., 40-50 pounds per cubic foot).
Elmendorf U.S. Pat. No. 3,164,511 (1965) disclosed a board composed of cross-cut wood flakes or strands oriented generally along the length of the board so that the average acute angle of the crossing strands was less than about 40.degree.. Elmendorf found, in tests using the softwoods Douglas fir and hemlock, that by controlling the orientation of the wood flakes or strands within the 40.degree. average strand crossing level, the modulus of rupture along the length of the board was equal to or greater than that of plywood for laminates having a specific gravity in the range of 0.65 to 0.90 (about 40-56 pounds per cubic foot), and incorporating about 4 percent of a phenolic resin binder.
Another approach to the construction of a wood composition panel is revealed by the Himmelheber U.S. Pat. No. 3,447,996 (1969). Himmelheber proposed a core of extruded adhesive coated wood chips or shavings oriented approximately normal to the plane of the panel and having surface layers of adhesive coated wood fibers transverse to the direction of the core fibers. Himmelheber noted that if hardwood chips were used for the core, the resulting board would be excessively heavy.
During the 1970's attempts were made to utilize certain of the hardwoods existing, for example, at the southern pine sites. The U.S. Forest Service was in the forefront of this work, and its research included tests of different types of flakers and studies of the characteristics of each of the various species of hardwoods found at the southern pine sites. A summary report by Hse et al., entitled "Laboratory-Scale Development of a Structural Exterior Flakeboard from Hardwoods Growing on Southern Pine Sites," was published in (25) Forest Products Journal, April 1975, pp. 42-50. This report concluded that for boards or panels having a density below 50 pounds per cubic foot which would qualify as a medium density panel, an addition of 20 percent of southern pine softwood was necessary even to come close to the specifications promulgated in 1973 by the U.S. Forest Service laboratory at Madison, Wisconsin for a medium density structural board. These specifications were:
______________________________________ Property Target or Goal ______________________________________ Modulus of Elasticity (MOE) in bending 800,000 psi Modulus of Rupture (MOR) 4,500 psi Internal Bond Strength (IB) Dry 70 psi After Accelerated Aging 35 psi (AA) (ASTM D1037) Lateral Nail Resistance (1/2" thick board) 300 lb. Nailhead Pullthrough (1/2" thick board) 250 lb. Nail Withdrawal from Dry Board 40 lb. Hardness 500-1200 lb. Linear Expansion (30-90% RH) 0.25% Thickness Swelling (30-90% RH) 8% Density 37-43 lb./ft.sup.3. ______________________________________
Further work by the Forest Service is reported in an article by Hse entitled "Exterior Structural Composite Panels with Southern Pine Veneer Faces and Cores of Southern Hardwood Flakes," published in (26) Forest Products Journal, July 1976, pp. 21-27. Although the wood flakes used in this work were carefully cut in a laboratory peeling machine to precise dimensions, visual observation of the samples shows horizontal (interlaminar or in-the-plane) shear failures and the data shows excessive thickness swelling in the 5-hour boil test (22-57 percent swelling) and in the vacuum-pressure soak (VPS) test (20-32 percent swelling). No specific data on the internal bond strength was reported, but the horizontal shear failures and the excessive thickness swelling indicate an insufficient internal bond strength.
A "fast-cure phenolic resin" developed by Hse and referred to in the April 1976 Forest Products Journal article, supra, is disclosed in the recent Hse U.S. Pat. No. 4,209,433 (1980). This patent suggested the use of an organic polyisocyanate/phenolic resin adhesive and described a method of first applying the organic polyisocyanate to the wood furnish and then applying the phenolic resin to the wood furnish. The use of this procedure was said to improve the internal bond strength of the product.
In addition to the above references, attention may be called to the Yan Canadian Pat. No. 584,444 (1959) which related to a method for providing uniform density to the layers of a particleboard. Elmendorf U.S. Pat. No. 3,478,861 (1969) disclosed a method of aligning flakes in the face layers of a particleboard but does not teach the production of a medium density board made from flakes of hardwood species.
As noted above, the several prior art medium density products have been formed from low density species and softwoods. Where attempts have been made to use mixed species of hardwoods, difficulties in bonding resulted. This was noted by Maloney in his book, Modern Particleboard & Dry-Process Fiberboard Manufacturing, Miller Freeman Publications, San Francisco (1977), at p. 161:
"The reason for preferential use of the relatively light species is that they can be compressed into medium-density particleboards with the assurance that sufficient interparticle contact area is developed during the pressing operation to achieve good bonding. Heavier species simply cannot be compressed into medium-density particleboards that are well bonded."
Hunt et al. U.S. Pat. No. 4,246,310 disclosed more recent work by the Forest Service in the structural particleboard area. The Hunt patent discloses a structural board for roof decking made solely from high density species of hardwood and does not suggest that mixtures of high and low density hardwood species having widely varying properties could be employed for roof decking or other purposes. The Hunt patent discloses no data concerning the effects of aging on the internal bond strength of the Hunt structural board. However, the Forest Service has established that the red oak structural boards, though adequate for roof decking, do not meet its own criteria for structural sheathing panels with respect to the internal bond strength retention. See Hunt, M. O., Hoover, W. L., Fergus, D. A., Lehman, W. F., and McNatt, S. D., 1978, Red Oak Structural Particleboard for Industrial/Commercial Roof Decking, Purdue University, Agricultural Experiment Station, Research Bulletin 954.